A linear type AC/DC power supply and a switching type AC/DC power supply are widely used. The linear type AC/DC power supply receiving a high voltage and a large current (more than 10 mA) needs mass heat dissipation, and the converting efficiency of the linear type AC/DC power supply is decreased. With an improved converting efficiency, the circuit structures of the switching type AC/DC power supply can be a non-isolated buck-boost circuit, an isolated flyback circuit, a non-isolated buck circuit, an isolated forward circuit, or a push-pull circuit. However, the isolated circuit cooperates with a mass transformer, and the ground in the non-isolated circuit is a fire wire of the alternating current, a neutral line, or a floating ground.